At highly sensitive real-time ion-assisted OH measurement technique has recently been developed and tested. It offers an OH detection sensitivity of about 1¿105 molecules/cm3 with a 300-s integration time and provides useful OH concentration measurements on a 10-s time scale. Measurements of OH concentration are seen to track solar flux during both short and long periods of cloudiness. Good temporal correspondence between changes in OH and O3 concentration as differing air masses pass the measurement site are also observed and discussed. The present technique provides a new opportunity for fast sensitive OH measurement using a totally new nonoptical measurement scheme. ¿1991 American Geophysical Union